1. Field of the Invention
The present invention relates to a piezoelectric ceramic transformer used in various types of power circuits for generating high voltage, and particularly to a highly reliable piezoelectric ceramic transformer which is thin and compact, and which also generates high voltage.
2. Description of the Related Art
In recent years, wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as television deflecting devices or charging devices of copiers which require for high voltage. Such electromagnetic transformers take the form of a conductor wound onto a core of magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are compact and slim in shape are extremely difficult to produce.
To remedy this problem, piezoelectric transformers utilizing the piezoelectric effect have been provided. FIG. 1 shows the construction of a Rosen-type piezoelectric transformer, a representative example of a piezoelectric transformer of the prior art. For producing high voltage, the portion indicated 41 in this piezoelectric ceramic-transformer is its low-impedance driver provided with electrodes 43, 44 on its upper and lower surfaces, this portion being polarized in the direction of the thickness of a piezoelectric plate as shown by the arrow 49 in the figure. The portion indicated 42 in the figure is a high-impedance generator provided with electrode 45 at its end, the generator 42 being polarized along the length of the piezoelectric plate as shown by the arrow 50 in the figure. This piezoelectric transformer operates as follows: When voltage is impressed to drive electrodes 43, 44 from external terminals 46, 47, an electric field increases in the direction of polarization, and a longitudinal vibration in the longitudinal direction is excited by the piezoelectric effect displaced in a direction perpendicular to polarization (hereinafter abbreviated "piezoelectric transverse effect 31 mode"), whereby the entire transformer vibrates. Moreover, in generator 42, due to the piezoelectric effect generating a potential difference in the polarization direction (hereinafter abbreviated "piezoelectric longitudinal effect 33 mode") which is caused by a mechanical stain occurring in the polarization direction, a voltage is produced which has the same frequency as the input voltage from output electrode 45 to external terminal 48. At this time, if the drive frequency is made equal to the resonance frequency of the piezoelectric transformer, an extremely high output voltage can be obtained. Furthermore, for inputting high voltage and outputting low voltage, the high-impedance section 42 of longitudinal effect can obviously be made the input side and the low-impedance section 41 of transverse effect be made the output side.
This piezoelectric transformer is used in a 10 resonant state, and compared with ordinary electromagnetic transformers, has numerous advantages including: 1) a compact and slim shape can be achieved because a wound-type construction is not required and energy density is high; 2) non-combustibility is possible; and 3) there is no electromagnetic induction noise. Nevertheless, in this Rosen-type piezoelectric transformer of the prior art, the electrode of the generator portion is located at the end of the transformer, i.e., at the loop of vibration, and external electric terminal of lead wire must also be led out from this portion. In such a case, because the mass of the terminal of the lead wire as well as connection component such as solder lies at the loop of vibration, there is an increase in mechanical loss and hunting of frequency characteristics during resonance. Increase in mechanical loss causes a-drop in efficiency, while hunting in frequency characteristic causes instability in circuit operation, both problems posing serious obstacles to putting such a transformer into practical use.
In addition, in contrast with a piezoelectric element used for signal processing such as in filters, a piezoelectric transformer must operate at relatively high power and is caused to vibrate at large amplitudes approaching the capacity limits of piezoelectric 10 materials. In such a piezoelectric transformer, the location of the connection components at the loop of vibration means that the connection components receive the brunt of large vibrations, with the result that, despite the use of connection methods such as soldering or bonding, the reliability of the connection components is severely compromised in terms of life expectancy.
There is the further disadvantage that, although a relatively high output voltage can be obtained when the load resistance value is significantly greater than the piezoelectric transformer impedance, when the load resistance value is not so large, a particularly high output voltage cannot be obtained.
Furthermore, as is clear from FIG. 1, there is the problem that this Rosen-type piezoelectric transformer is of three-terminal construction and electrical insulation between the input and output cannot be achieved.